C. P. FENIMORE AND G. W. JOSES
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Voli 65
RATE OF REACTION OF METHANE WITH H ATOMS AND OH RADICALS IN FLAMES BY C. P. FENIMORE ASD G. W.JOKES General Electric Reseaf-ch Laboratory, Schenectady, IT.Y . Received June 28. 1961
It wm assumed that the irreversible formation of methyl radicals in low pressure flames of CH4-Hz-O2-N20-Ar occurred a t the rate [CH4](kl[H] k2[OH] k 3 [ 0 ] ) = -d[CH4]/dt and k l , ke and k Bwere estimated from the variation of. -d[CH41 / dt with [HI, [OH] and [O]. The reaction with 0 atoms was not very Important under the experimental conditions, and k~ was no larger, therefore, than about 2 X l O Q 1. mole-' see.-' at 1600°K. In fuel-rich flames, JOH]/[H] = 0.06 to 0.2, and the attack of H atoms predominated; but in lean mixtures, where [OH]/[H] was about 40 times larger, an attack by OH radicals was most important. The constants obtained were kl = 2 3: lO11e-11.5'RT 1. mole-' set.-', kz = 3.5 X 10'le-Q'RT 1. mole-' set.-', with an uncertainty of 1.5 kcal. in each activation energy.
+
+
Introduction It is convenient to consider that methane burns in two stages. I n one stage, methyl radicals are formed by one or more of the reactions
+ CH,
kl
+ HQ
( 1)
+ CH, e CH3 + HzO 0 + CH, z CHa f OH
(2)
H
CH3 k-
1
OH
(3)
and oxidized a t a rate k4[O][CH3]to yield carbon monoxide, and hydrogen or water 0
k, + CH3 + . . . +eventually CO + . . .
(4)
Although the intermediate steps of (4) are uncertain, k4 is known approximately; it is so large that in steady, low pressure flames, where [O] is generally a few per cent. of [02], the oxidation of methyl radicals by molecular oxygen is not very imp0rtant.l Another stage, overlapping the first, is described by the known reactions of OTCO-H~ flames
+ 02 zOH + 0 + CO e COn + H OH + HZO + H 0 + H2 J _ OH + H H
( 5)
OH
( 6)
+
and an estimate of [CH,] in the reaction zone can be obtained from -d[CH,]ldt and [O]. Also, the reverse of (1) is negligible if
> k4 [O][CH3];or substituting for [Hz], [O] and IC4, from the condition that IC-' >> 0.2 X lo9a t 1800°K. Using the kl/k-' given above, = our estimate of kl gives a reverse constant of 0.9 X lo9, which is large enough to permit a considerable degree of eqilibration; but a kl smaller by a factor of ten would be inconsistent with such an equilibration either in the first run of Table I, or in a number of rich flames studied previously which also were more or less equilibrated.' Our lc2 agrees very well with other work. 10" Avramenko and Lorentsolo found kt e-8J/RT a t much lower temperatures; and Westenberg and Fristrom3 showed that if methane reacted uia (2) in very lean flames a t 1650 to 1840", kz must be about 2 X 1O'O 1. mole-' set.-'. They were not sure that (3) could be excluded, but it is not very important in our work and we find kz between 1 and 3 X lolo. Westenberg and Fristrom estimated [OH] from reaction 6 with a value of ke about 40% smaller than ours a t 1760°, and therefore might have been expected to deduce rather smaller ke than ours. Also they neglected the reverse of (6), claiming that the omission would introduce little error, and our measurements agree with this claim. In our lean reaction zones, the reverse of (6) varied from about 201, of the forward reaction in the 1580" measurement to 20% of the forward reaction in the 1800" measurement. If they had incurred a little error by omitting the reverse of (6), it would have tended to compensate for the difference between their ks and ours.
-
(10) Quoted by N. N. Semenov, "Some Problems of Chemical Reactivity," Vol. 2, Pergamon Press, London, 1959. p. 104.
